Nelson Mandela has spent a second night in hospital where he is receiving treatment for a lung infection.
The 94-year-old former South African president was admitted to a hospital in Pretoria in the early hours of Saturday in a "serious but stable condition", according to the office of the current president, Jacob Zuma. His family was at his bedside.
Mr Mandela has a history of problems with the illness and has been admitted to hospital several times this year.
People across South Africa held prayers for the former leader and some left messages outside his home in Johannesburg.
His wife Graca Machel cancelled an appearance at a high-profile hunger summit in London to be with her husband.
Ms Machel was due to address the Nutrition for Growth meeting in the capital along with dozens of African leaders, non-governmental organisations and charities, as well as speakers such as Bill Gates and Justine Greening, Secretary of State for International Development.
Mac Maharaj, a spokesman for Mr Zuma's office, said: "During the past few days former president Nelson Mandela has had a recurrence of lung infection. This morning at about 1.30am his condition deteriorated and he was transferred to a Pretoria hospital. He remains in a serious but stable condition.
"The former president is receiving expert medical care and doctors are doing everything possible to make him better and comfortable. President Jacob Zuma, on behalf of government and the nation, wishes Madiba a speedy recovery and requests the media and the public to respect the privacy of Madiba and his family."
The anti-apartheid leader became president in South Africa's all-race elections in 1994.
Mr Maharaj said he first developed his lung condition many years ago when he was incarcerated in prison, and that it is a problem that "comes and goes". "It gets treated and he gets better," Mr Maharaj said. "He's prone to lung infection. There's a long history behind that. He's suffered from lung infection at different times. The first time we came across it was years ago when he was in prison."